Hepatitis B virus (HBV) is a major cause of chronic viral hepatitis that increases dramatically the risk of liver cancer and other end-stage liver diseases such as cirrhosis. HBV belongs to the Hepadnaviridae, a family of para-retroviruses that have a small DNA genome and replicate through an RNA intermediate (the pregenomic RNA, or pgRNA), by a unique reverse transcription pathway. Hepadnaviruses initially assemble an immature nucleocapsid (NC) composed of multiple copies of the capsid protein, and the packaged pgRNA and the virally encQded reverse transcriptase (RT). The immature NC undergoes a process of maturation whereby pgRNA is converted to the characteristic, partially double stranded (OS), relaxed circular (RC) DNA by RT-mediated reverse transcription. The RC DNA-containing, mature NC is then selectively enveloped with the viral envelope (surface) proteins and secreted extracellularly as virions. Using recently established cell-free and cell culture systems and newly developed approaches and models, we propose (1) to determine the nature of nucleic acid within NC that blocks or triggers NC envelopment; (2) to determine the maturation-associated NC structural changes and their role in NC envelopment; and (3) to determine the role of host factors in regulating selective NC envelopment. These Revised Specific Aims are derived from the three sUb-aims of Specific Aim 3 as proposed in the original application. These stUdies are selected because of their likelihood for successful completion within the two year time-frame supported by the the American Recovery and Reinvestment Act of 2009 funding. Also, they still constitute a coherent and sound project separate from Specific Aims 1 and 2 of the original application. The elucidation of the molecular basis of, and viral and host factors involved in, selective NC envelopment will not only provide important insights into the mechanisms of hepadnavirus replication and virus-host interactions but may also facilitate the development of novel and effective anti-HBV strategies targeted at these factors.